The power recently went out in my garage. When I went to the garage and flipped on the light, there was a small spark from within the switch box (where the wires enter the garage) and then no power. Note that my garage was built in 1950, and the wiring is a bit scabby. The garage breaker is a 15A that is in a sub-panel in my basement.

I opened up the wall, and the wiring is brought in and split in two branches: one to the outside receptacle, and one to the lights + a receptacle.

The hot is connected to 3 other wires (via wire nut), one to the light switch, one in parallel to the lighting (or receptacle...I'm not too sure), and one to the out door receptacle.

The Neutral is connected to 2 other wires (via a wire nut), one to the lighting and one to the out door receptacle.

The ground is grounded directly to the light switch body.

I have found that there is correct voltage between the hot and the Neutral only when: 1) There is no load on the out door receptacle 2) The lighting Neutral is not in contact with the garage service neutral

Basically, the voltage is only correct if the entrance Neutral has no load on it from either branch. When there is a load, the voltage goes to zero.

Does anybody know what's going on?

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  • That is confusing. Could it be you are not experienced using a DMM? Is the DMM set on AC (vs. DC)? Did you use the same reference terminal for all voltage measurements?
    – wallyk
    Aug 12 '16 at 3:40
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    I suppose I'm not as experienced as some, but I have quite a bit of experience. It is set to AC, and the same terminals were used (and validated on a known working source). I also switched from my cheapo multi-meter to my Fluke 1587 to verify, and it gave the same readings. Aug 12 '16 at 4:21
  • Okay, thanks. Just trying to eliminate a source of potentially misleading information.
    – wallyk
    Aug 12 '16 at 4:28

Ok ,you megged it's you know more than most home owners.

This sounds like a broken conductor.

With a light load everything works. Add a few amps and the arc separates the wires. You have megged it so you know it is not going to ground. The only other problem that I have seen like this is from "back stabs" or broken conductors.

With the age of your home I would look for a broken wire in an outlet that daisy chains to the next outlet.

That or a problem in the panel.

After reading everything again your Fluke meter may be the problem with "leakage" or "phantom" voltages. I can say this as a profesional that uses many diferent models of fluke meters & meggers.

Very high impeadance meters will show a normal voltage with no real coper to copper connection.

Some times the voltage is from leakage on solid state electronics. Other times I have found moisture & dust enough to cause phantom voltages. Once a load is on the circuit the voltage is gone.

  • 1
    Good call. I rigged up a suicide cord, and ran power into the garage on the load side of the entrance conductors, and everything turned on, so that put the break on the service side of the garage. I put a load off the breaker, and the load turned on, so that puts it between the breaker and the garage...underground.... At this point I would just like to put out a friendly reminder that using a slightly larger conduit for the future's sake is always nice. It looks like I am going to have to pull new conductors into the garage (underground). I will in the morning! Aug 12 '16 at 7:22
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    @Freddyjones39 The phantom voltage problem can be caused by a variety of things but is usually inductive coupling, can really only be avoided by using a low impendence tester. I carry a "Wiggy" for exactly that reason... if you do much electrical work it's a good investment.
    – PaulBinCT2
    Aug 12 '16 at 12:04
  • @Freddyjones39 I'm always interested in learning new troubleshooting methods and techniques. I'd like to hear more about the suicide cord. What is it? I'm guessing just an extension cord with the female end cut off to hook into one part of the other circuit at a time? Jul 12 '18 at 2:01

My first theory is that you're using a typical digital multimeter and the "line voltage' you're seeing is a phantom... you don't really have voltage there in the true sense.

I think you had a short somewhere, hence the spark from your switch when you closed the circuit. The short burned a conductor or connection through hence your current (get it? ;) ) lack of power.

  • I just switched from my "cheapo" Klein to my Fluke 1587. I don't know if it is better at avoiding phantom voltages (I really don't have any experience with them) but it did give me the same results, and the voltages do match the voltages that are at initiation of the conductors at the sub-panel in the house. Aug 12 '16 at 3:55
  • I also did a megger test while I was at it, and all the lines seem to be reasonably insulated from each other (and from the EMT conduit that they are contained in). Interestingly enough, there is a small sound maker to help get rid of mice, and it does work (at least the light is on) when the wires are all connected. Nothing else does though. That is plugged into the indoor outlet. Aug 12 '16 at 3:57

Here's another trouble shooting idea. You can use the suicide cord to test each of the original feeder conductors individually under load. First disconnect the black feeder wire at the house and the white conductor at the suicide outlet. If all works well, then the original white is successfully working. Then use a similar strategy to test the original black feeder. You can even test the original ground to see if it will work as a neutral, might be dangerous though. Since you have conduit, you will only need to replace the fried conductor, not all three.

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